The digital phase shift keying of a sinusoidal signal (PSK) is one of the most efficient modulation techniques, both in terms of noise immunity and required bandwidth. Nevertheless, the systems for demodulation of PSK signals proposed by the state of the art are quite complex. Therefore, other less efficient digital modulation schemes are usually preferred because of their simpler demodulation, for instance Frequency Shift Keying (FSK) or Amplitude Shift Keying (ASK).
The simplest PSK signal is the Binary PSK signal (BPSK). In this case, the carrier phase is shifted between two possible states, 0° and 180°, according to the bit stream. BPSK signals can be easily obtained by multiplying the carrier by +1 (0° phase state) or by −1 (180° phase state). From the receiver point of view, it is impossible to know if the phase of an incoming BPSK signal corresponds to 0° state or to 180° state. This is due to the fact that the actual propagation path from the emitter to the receiver is usually unknown. To avoid this indetermination, the information to be transmitted is coded as transitions between phase states, instead of being coded as fixed phase values. Therefore, when a logic “1” has to be transmitted then the phase of the carrier signal is shifted, whereas the phase is unchanged for a logic “0”, or vice versa. The signal coded in this way is known as Differential BPSK (DBPSK). It should be noted that from the signal point of view there is no difference between BPSK and DBPSK. The only difference between them is the pre-processing (at the transmitter side) or post-processing (at the receiver side) of the base-band signal. FIG. 1 shows the generation of BPSK or DBPSK signal as the product of the Base-band signal (derived from the bit stream or from the processed bit stream) and the sinusoidal carrier at the desired frequency.
The usual procedure for demodulating BPSK signals is that of coherent demodulation. Basically, the demodulation process consists of multiplying the received signal by a reference signal at the same frequency as the original carrier.
A detailed description of said coherent demodulation functioning as well as its main drawbacks are included in WO-A-03079624, property of the same assignee than the present invention, which whole content is incorporated herein by reference.
WO-A-03079624 was proposed to overcome the by then state of the art, and the invention there described presents the advantages of coherent demodulation (input signal tracking and demodulation process which is independent of the modulating signal bit period), but without the requirement for the explicit use of a frequency and phase locking loop (PLL or Costas loop). In essence, the system proposed in WO-A-03079624 is a converter of digital phase modulation (PSK) signals into digital amplitude (ASK) signals. ASK modulation is the simplest modulation scheme, both from the signal generation point of view and its demodulation, however, it is not very efficient with regards to noise immunity. The information contained in an ASK signal is transmitted by modifying the amplitude between two pre-established values. Demodulation of these signal is very simple as it only requires an envelope detector, for example, a diode and lowpass filter, followed by amplification and/or signal regeneration as necessary (FIG. 1). The simplicity of ASK signal demodulation makes it useful to have signal converters available with a more efficient modulation with respect to noise, for example, FSK or BPSK and ASK signals.
The invention proposed by WO-A-03079624 concerns to a system and method for the conversion of digital phase modulated (PSK) signals into digital amplitude modulation (ASK) signals by using a power divider with an input injected with a PSK signal, the outputs of which are connected to at least two argument/frequency dividers having each one a natural tuneable resonant frequency, the output signals of which have a phase difference depending on the phase changes of the input signal, which for example is of 0° or 180° in the case of BPSK conversion or of 90°, 180° and 270° in the case of QPSK conversion.
The argument/frequency dividers outputs are connected to the inputs of at least a power combiner, the output of which is the sum of the at least two argument/frequency dividers output signals. In the case of a BPSK to ASK converter (see FIG. 2) when the phase difference between the two signals entering the power combiner are, for example, 0°, additive interference takes place in the combiner, and when this difference is, for example, 180°, a subtractive interference occurs, so that the result is an ASK signal.
The proposed system is extrapolable to phase modulation with a greater number of symbols, not only to the mentioned QPSK signals but also to M-PSK modulation.
The explanation of the argument/frequency dividers locking phenomenon of both frequency and phase of its circuits when injected with a signal having a frequency close to the second harmonic of its fundamental resonant frequency, which are the principles in which said invention (and also the present invention) are based, is included in the WO-A-03079624 description, which, as it was said above, is wholly incorporated herein by reference.
Although the proposal of WO-A-03079624 represented a step forwards from the state of the art, its performances and results can be enhanced particularly with respect to avoid mutual locking between the argument/frequency dividers of the converter proposed, something that the power combiners there utilized don't successfully achieve, and also with respect to obtain a complete conversion to base band, i.e. the demodulation of the PSK signals, without the needing of additional circuitry (such as the mentioned envelope detector), and not only its conversion to ASK signals.